The Collaborative International Dictionary of English v.0.48:

Derby \Der"by\ (?; usually ? in Eng.; 85), n.
   1. A race for three-old horses, run annually at Epsom (near
      London), for the Derby stakes. It was instituted by the
      12th Earl of Derby, in 1780.
      [1913 Webster]

   Derby Day, the day of the annual race for the Derby stakes,
      -- Wednesday of the week before Whitsuntide.
      [1913 Webster]

   2. A stiff felt hat with a dome-shaped crown.
      [1913 Webster]

WordNet (r) 3.0 (2006):

derby
    n 1: a felt hat that is round and hard with a narrow brim [syn:
         bowler hat, bowler, derby hat, derby, plug hat]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "Derby":
   Kentucky Derby, Preakness Stakes, chase, flat race, handicap,
   handicap race, harness race, horse race, hurdle race, plate race,
   purse race, quarter-horse race, stake, stake race, steeplechase,
   sweep, sweepstake, sweepstakes, trotting race



Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "derby":
   Le Mans, Olympic games, Olympics, air race, automobile race,
   bicycle race, boat race, bout, concours, contest, contest of speed,
   cross-country race, dash, dog race, drag race, encounter,
   endurance race, engagement, fight, footrace, game, games, go,
   gymkhana, heat, hurdle race, joust, lap, marathon, marathon race,
   match, match race, matching, meet, meeting, motorcycle race,
   obstacle race, potato race, race, rally, regatta, relay,
   relay race, rencontre, road race, run, sack race, speedway race,
   sprint, sprint race, stock-car race, test, three-legged race, tilt,
   torch race, tournament, tourney, track race, trial, walk,
   yacht race



U.S. Gazetteer Places (2000):

Derby, CO -- U.S. Census Designated Place in Colorado
   Population (2000):    6423
   Housing Units (2000): 2145
   Land area (2000):     1.638168 sq. miles (4.242835 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    1.638168 sq. miles (4.242835 sq. km)
   FIPS code:            20275
   Located within:       Colorado (CO), FIPS 08
   Location:             39.838785 N, 104.917082 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Derby, CO
    Derby


U.S. Gazetteer Places (2000):

Derby, CT -- U.S. city in Connecticut
   Population (2000):    12391
   Housing Units (2000): 5568
   Land area (2000):     4.981069 sq. miles (12.900910 sq. km)
   Water area (2000):    0.401503 sq. miles (1.039887 sq. km)
   Total area (2000):    5.382572 sq. miles (13.940797 sq. km)
   FIPS code:            19480
   Located within:       Connecticut (CT), FIPS 09
   Location:             41.323005 N, 73.084331 W
   ZIP Codes (1990):     06418
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Derby, CT
    Derby


U.S. Gazetteer Places (2000):

Derby, IA -- U.S. city in Iowa
   Population (2000):    131
   Housing Units (2000): 63
   Land area (2000):     0.263253 sq. miles (0.681822 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.263253 sq. miles (0.681822 sq. km)
   FIPS code:            20125
   Located within:       Iowa (IA), FIPS 19
   Location:             40.930972 N, 93.456913 W
   ZIP Codes (1990):     50068
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Derby, IA
    Derby


U.S. Gazetteer Places (2000):

Derby, KS -- U.S. city in Kansas
   Population (2000):    17807
   Housing Units (2000): 6407
   Land area (2000):     7.445740 sq. miles (19.284376 sq. km)
   Water area (2000):    0.011333 sq. miles (0.029352 sq. km)
   Total area (2000):    7.457073 sq. miles (19.313728 sq. km)
   FIPS code:            17800
   Located within:       Kansas (KS), FIPS 20
   Location:             37.552407 N, 97.261492 W
   ZIP Codes (1990):     67037
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Derby, KS
    Derby